Did you know that H1N1 reached Russia by whale? Or that you can get the flu from a bird and give it to a pig? Find out why viruses tend to jump from animals to us, and why they seem to always pick the cute ones.

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Viruses have numbers on their side, but not anything else. They're brainless, defenseless, reproductive system-less little bits of organic matter that some people don't even believe count as life. And yet they can jump not only between people but between entire species. They do so especially frequently between specific species, even when those two species aren't very much alike, like birds and humans. Let's take a look at how viruses species-hop, and why they find certain pathways so convenient.

The Animal Laboratories

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The most well-studied instances of a zoonosis — animal viruses transferable to humans — involve the influenza virus. In 1918, migrating birds spread an epidemic that killed more people than World War I. In 1957, a new kind of influenza appeared on the horizon, again killing millions. The latest scare was the relatively over-hyped H1N1 influenza virus, dubbed swine flu. Although only the last bout of influenza specifically mentioned pigs, scientists believe that they were responsible for much of the carnage.

Each species has their own set of influenza viruses, and birds and humans can infect each other, but pigs have the ability to get viruses from both humans and birds. They're like the mix-masters of the disease world. When a very unfortunate pig has two of the types at once, the viruses inside them can swap variations of two very important proteins. This new virus can be transmitted directly to humans, or can be transmitted to birds.

The H protein is called haemagglutinin and the N protein is neuraminidase. Theire are sixteen variations of the H protein and nine of the N protein. Each influenza virus also has six other important packets of protein, but of all of them, the H is the protein that the body develops antibodies against. Although H1N1 has popped up recently, the two proteins have been in circulation before. Prior to 1957, an H1N1 virus went through the population for years. In 1957, a new version of the virus, H2N2, emerged from some infected pigs. The resulting flu devastated a population that only had antibodies designed for H1N1. When a new version of H1N1 appeared in the 2000s, this time as the result of two pig viruses swapping proteins with no help from the birds, this history with the H protein was why pre-1957 population was far more well-placed to withstand the outbreak. As long as they still had the antibodies in their system, they could fight the flu fast.

What Animals Can Kill You?

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Not only can a bunch of adorable animals get you sick, they can do it in many different ways. Animal dung, animal saliva, and uncooked animal meat find their way into and onto a great deal of things, even if you're not directly handling the animals themselves. Insects that bite multiple animals spread diseases, too. And of course, humans themselves are happy to help the spread of disease. An outbreak of avian flu (H5N1), was stopped when an animal smuggler was discovered trying to get a live, infected Thai eagle out of the country. An outbreak of H1N1, a different variation, in the USSR in 1977 was traced back to the harvesting of infected whales. Famously, HIV was thought to have crossed over to humans when people ate infected primate meat. Monkeypox came over on infected rats, who then infected prairie dogs that were sold as pets. Lions and tigers, exported illegally as pets, can be infected with avian flu. (There was even a disease called Seal Finger, which was prevalent among those who hunt seals and polar bears, which caused joints to swell up so much that they became unusable, and had to be amputated.)

We also find indirect ways to spread diseases from animals to humans. Farming puts people in direct contact with some animals, but also ushers in incidentally. Hemorrhagic fever exploded in Argentina without any explanation, until it was revealed that the carrier, the corn mouse, was able to expand its range drastically, being suited to the environment of the expanding farms in the region. Cities encourage small pests that would be picked off in the wilderness. Even being a foodie can get people sick. When reports of hepatitis E started popping up in rich countries, instead of the poor countries to which it had been mostly confined, researchers were surprised. It was only when they learned that people had been eating the liver of wild boars — because it was what was available or because it was a delicacy, depending on your country and economic status - that they understood the isolated explosions of the disease.

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Reverse a Zoonosis

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Humans are known for giving as good as we get, in terms of ruination. That's certainly true with a zoonosis. The modern version of H1N1 cropped up in a Canadian group of pigs. When people traced the source, looking for some kind of intermingling of pigs from Mexico and pigs from Canada, they found only a human, who had been to Mexico, picked up the virus, flown up to Canada, and apparently coughed on some pigs. A chimpanzee in a zoo died of a human metapneumovirus that must have been brought in by a visitor. Visitors in a bonobo refuge infected six bonobos with influenza. The Los Angeles Zoo and the San Francisco Zoo have both had to contend with elephants that contracted tuberculosis.

Mostly, though, the infection pathway isn't direct. If keeping animal droppings off of human food is a problem, the reverse is an even greater one. Forget chimps, humans are the species that smears their crap everywhere. And animals don't have access to soap. Perhaps, considering they can incubate what we give them and, under the right circumstances give some very nasty stuff back to us, it's time to take a look at exactly how much we cross over with the animal world, and what the consequences of that crossover will be.